1. Field of the Invention
Embodiments of the invention relate to methods and apparatuses for manufacturing a semiconductor device, and, in particular, to methods and apparatuses for manufacturing thin semiconductor devices such as power semiconductor devices used in power conversion devices.
2. Description of the Related Art
In power semiconductor devices represented by IGBTs (insulated gate bipolar transistors), methods are being developed to reduce the thickness of semiconductor substrates or wafers for producing semiconductor devices to achieve high performance of the devices. In addition, wafers with enlarged diameters are being developed to increase the number of power semiconductor elements that can be formed of a sheet of wafer, thereby reducing the costs of the semiconductor devices.
The reduction in wafer thickness, however, in certain circumstances, may cause a break at the peripheral edge of the wafer resulting in chipping of the wafer. Because thin wafers can show lowered mechanical strength, a problem can also arise that the wafer is liable to generate cracks, breakage, and deflection. These problems are noticeable in wafers with a large diameter, in particular. Thin and large diameter wafers showing large deflection have difficulties in transporting the wafer between manufacturing steps and in positioning the wafer on manufacturing devices.
The manufacturing process of vertical power semiconductor devices typically includes the steps of ion implantation, heat treatment (annealing), metallic film deposition, etc., also on the back surface of the wafer. Hence, the above-mentioned problems can cause difficulty in carrying out these steps on the back surface of the wafer.
In order to solve or minimize such problems, a wafer has been proposed that has a ring-shaped stiffening portion at the periphery of the back surface side of the wafer to reinforce the wafer having a reduced thickness. The wafer having a ring-shaped stiffening portion has the periphery of the back surface side thicker than the central portion of the wafer. Use of the wafer having a ring-shaped stiffening portion substantially reduces warp and deflection of the wafer and enhances strength of the wafer in handling the wafer in the transport step, preventing the wafer from generation of breakage and chipping.
FIGS. 12A through 12D show shapes of a wafer having a ring-shaped stiffening portion. FIG. 12A shows a wafer with an orientation flat 110 having a ring-shaped stiffening portion 111 at the periphery of the wafer.
FIG. 12B shows a wafer with a notch 120 having a ring-shaped stiffening portion 121 at the periphery of the wafer.
The inner portion of both the wafers inside the ring-shaped stiffening portion is a region for forming semiconductor device elements.
Japanese Unexamined Patent Application Publication No. 2007-173487 (also referred to herein as “Patent Document 1”), for example, discloses a method for fabricating a wafer having the ring-shaped stiffening portion. The method uses a grinding apparatus provided with a grinding member having a diameter smaller than that of the wafer and grinds the central portion of the wafer thin leaving the peripheral portion of the back surface side of the wafer to form a rib.
FIGS. 13A and 13B are simplified sectional views showing a grinding step in the process of fabricating a wafer having the ring-shaped stiffening portion.
The following describes a step of fabricating a wafer having the ring-shaped stiffening portion with reference to FIGS. 13A and 13B. The wafer 20 is first set in a cassette (not depicted) of a grinding apparatus. The wafer 20, after positioning by a transport robot or the like, is then transported above a chuck table 10 and put on the surface of an attachment plate 12 of the chuck table 10. The chuck table 10 is connected to a vacuum system (not depicted) that supplies a negative pressure through the attachment plate 12 to attract and hold the wafer 20 as shown in FIG. 13A. The attachment plate 12 is made of porous ceramics, for example.
The grinding apparatus is provided with a grinding member 133 having a diameter smaller than that of the wafer. The grinding member 133 has a grindstone on the contact surface with the wafer. The grinding member 133 rotates on its own axis and the axis itself turns around on the wafer to grind the central portion of the wafer.
The grinding apparatus grinds only a central portion of the wafer leaving the peripheral portion with just the thickness as of the original wafer inputted to the grinding apparatus. Thus, a wafer 22 is fabricated having the central portion ground to a thin desired thickness as shown in FIG. 13B. FIG. 12C shows a cross-section of a thin fabricated wafer, which has a ring-shaped stiffening portion 122 (a rib structure) at the periphery of the wafer.
FIGS. 14A, 14B, and 14C are sectional views of essential parts in another example of grinding step in the process for fabricating a wafer having the ring-shaped stiffening portion.
The grinding apparatus of FIGS. 14A, 14B, and 14C is provided with two grinding members 131 and 132 containing abrasive grains of different grain sizes, whereas the grinding apparatus of FIGS. 13A and 13B has a single grinding member 133.
In the grinding step of FIGS. 14A, 14B, and 14C, a semiconductor wafer 20 before grinding, which can be an eight inch wafer having a thickness of 725 μm, for example, after positioning by a transport robot (not depicted) or the like, is transported on a chuck table 10 and held by suction on the surface of an attachment plate 12. In the first grinding step, as shown in FIG. 14A, the central portion of the wafer 20 is ground using a grinding member 131 that is provided with a grindstone containing abrasive grains having a relatively large average grain size. The grinding step is conducted on the central portion of the wafer 20 down to a predetermined remaining thickness of 100 to 150 μm, for example, leaving the peripheral portion with a width of 1 to 5 mm, for example. After the central portion is ground to the desired thickness, the second grinding step is conducted, as shown in FIG. 14B, using a grinding member 132 that is provided with a grindstone containing abrasive grains having a smaller average grain size than that of the grindstone provided on the grinding member 131. The second grinding step grinds the back surface of the wafer processed by the first grinding step down to a predetermined thickness of 60 to 120 μm, for example, on the region with an inner circumference diameter smaller than that of the recessed inner circumference formed by the first grinding step. Thus, a wafer 23 having a ring-shaped stiffening portion is fabricated as shown in FIG. 14C. Here, the second grinding step grinds the wafer to an inner diameter that is smaller than the recessed inner diameter formed in the first grinding step, because positioning accuracy of the grinding machine used in the second grinding step is taken into consideration Thus, the grinding member 132 used in the second grinding step does not become in contact with the side wall of the ring-shaped stiffening portion that is formed in the first grinding step.
The wafer is ground only on the central portion leaving the peripheral portion of the wafer. Thus, a wafer 23 as shown in FIG. 14C is fabricated that is machined to a desired thickness only on the central portion of the wafer, leaving the peripheral portion with the thickness as inputted into the grinding apparatus. FIG. 12D is a section view of the wafer having a ring-shaped stiffening portion (a rib structure) 123 at the periphery of the wafer.
The wafer having the ring-shaped stiffening portion formed at the peripheral region of the wafer is transferred to the next step for cleaning and drying. A transport device, not depicted in FIGS. 13A, 13B, 14A, 14B, and 14C, picks up the wafer from the attachment plate 12 of the chuck table 10 and transported to the predetermined destination.
For transporting a wafer having a ring-shaped-stiffening portion (a rib structure), Japanese Unexamined Patent Application Publication No. 2009-059763 (also referred to herein as “Patent Document 2”), for example, discloses a construction in which the upper surface of the ring-shaped portion is attracted by evacuation suction to transport the wafer.
In the process of grinding by a grinding apparatus to form the ring-shaped stiffening portion at the periphery of the wafer, the process is generally conducted in consecutive steps.
After a grinding step is finished, the wafer having the ring-shaped stiffening portion (a rib structure) is removed from the chuck table and transferred to the next step of cleaning and drying. At the same time, the attachment plate on the chuck table is cleaned with a blush or the like, and the next wafer to be ground is supplied and held by suction on the chuck table.
FIGS. 15A, 15B, and 15C show conventional apparatus and process for transporting a wafer. A wafer 22 after completion of the grinding step and having a ring-shaped stiffening portion formed in the step is removed from the chuck table 10 and transferred to the next step by a transport device 80. The transport device 80 is provided with an attracting member 81 at the lower end of a support member 82 and transmits a negative pressure from a vacuum system (not depicted) to the bottom surface of the attracting member 81 through a supply and exhaust system (not depicted) inside the support member 82. The attracting member 81 attracts the thin central portion of the wafer 22 by suction as shown in FIG. 15A. The wafer 22 is removed from the chuck table 10 by elevating the support member 82 with the attracting member 81 attracting the thin region of the wafer 22.
While a wafer may be attracted only at the ring-shaped stiffening portion as disclosed in Patent Document 2, enough attraction so as to pick up and transport the wafer requires a flat and enough area of the upper surface of the ring-shaped stiffening portion to be attracted. The large area of the flat upper surface decreases the area of the central portion of the wafer, which is a device-forming region.
Attracting the wafer at the central portion thereof as shown in FIG. 15A ensures an area necessary for attracting the wafer and transportation without failure.
Alternatively, as shown in FIG. 16A, the wafer after completion of the grinding step having a ring-shaped stiffening portion can be held at the outer peripheral end of the wafer by a holding member 92 provided at the end of an arm 91 of a transport device 90. The arm 91 of the transport device 90 is movable so that the holding member 92 can approach and leave the ring-shaped stiffening portion of the wafer 22. The holding member 92 moves from a position apart from the ring-shaped stiffening portion and holds the wafer 22 at the ring-shaped stiffening portion thereof. The arm 91 is then elevated while holding the ring-shaped stiffening portion of the wafer 22 with the holding member 92 to remove the wafer 22 from the chuck table 10. This procedure allows the wafer 22 to be transferred to the next step without touching the thinned portion of the wafer 22.
The process including the grinding step needs to be consecutive. The total time required for the consecutive process should be decreased, the consecutive process on the wafer including: transfer, placing, grinding, picking up, cleaning of the chuck table, and transfer to the next wafer. In the step of picking up the wafer to transfer to the next step, in particular, two problems must be solved simultaneously: to shorten the time for picking up the wafer and to prevent the wafer from any damage.
The chuck table 10 has the cleaning water dropped on the wafer in the grinding step and the cleaning water used in the cleaning process of the chuck table 10 remained on the attachment plate 12 (FIG. 15A) or a combined attachment plate of 13 and 14 (FIG. 16B). The attachment plate 12 (FIG. 15A) or a combined attachment plate of 13 and 14 (FIG. 16B) is referred to as the attachment plate 12 or 13 and 14′ in the following.
If the cleaning water is remained between the wafer 22 and the attachment plate 12 or 13 and 14, the wafer 22 is adhered to the attachment plate 12 or 13 and 14 due to the surface tension of the cleaning water. In the grinding step for forming the ring-shaped stiffening portion, a negative pressure is supplied onto the attachment plate 12 or 13 and 14 from a vacuum system (not depicted) through a supply and exhaust path 11. After the grinding step, the negative pressure is released from the attachment plate 12 or 13 and 14 on the chuck table 10. However, after the release of the negative pressure, the presence of the cleaning water still inhibits the progress of air leakage through the porous attachment plate 12 or 13 and 14 of the chuck table 10 and causes the wafer 22 remaining adhered onto the attachment plate 12 or 13 and 14. If the wafer 22 were left on the attachment plate 12 or 13 and 14 for a long period of time for example ten minutes, after the release of the negative pressure, the air leakage through the attachment plate 12 or 13 and 14 would be advanced and the wafer 2 would become easy to be removed. This means, however, leaves the problem to reduce the time required for picking up the wafer unsolved and lowers the efficiency of the transport operation.
If the wafer 22 adhered to the attachment plate 12 or 13 and 14 were forced to be separated, the thin wafer 22 vulnerable to break would cause chipping and cracking. Thus, the problem to pick up the wafer with no damage is left unsolved.
The time duration for picking up the wafer 22 can be shortened by supplying, after release of the negative pressure, water, air, or a mixture thereof (indicated by the reference symbols 70 and 71 in FIGS. 15B, 15C and 16B) to the attachment plate 12 or 13 and 14 on the chuck table 10 through the supply and exhaust path 11. A positive pressure is given to the surface of the attachment plate 12 or 13 and 14 on the chuck table 10 by blowing up the water, air, or the mixture thereof (70, 71) through the attachment plate. The wafer after releasing adhesion to and separation from the attachment plate on the chuck table 10 is transferred to the next step by the transport device (80 in FIG. 15A, 90 in FIGS. 16A and 16B).
The blowing up of the water, air, or a mixture thereof 70, 71 shown in FIGS. 15B, 15C, and FIG. 16B causes deformation in the wafer 22. Stress concentration is liable to be developed especially at places damaged by the grindstone in the grinding step and places with varied curvature. Such places are indicated by the symbol 223 in FIG. 13B and the symbol 233 in FIG. 14C.
The deformation of the wafer 22 is very likely to occur at the part B indicated in FIG. 15C where the wafer 22 is pushed by the attracting member 81 of the transport device 80 and at the part B indicated in FIG. 16B where the wafer 22 varies in thickness thereof.
The wafer 22 undergoes occurrence of cracks on the ground surface thereof due to such deformation that makes the wafer locally float apart as indicated by the arrow A in FIG. 15C and FIG. 16B. Thus, there is a need in the art for an improved method and apparatus for manufacturing semiconductor devices.